The combination in Nb.sub.3 Sn of brittleness and superior superconducting properties has stimulated a number of prior art attempts to devise methods enabling fabrication of various forms, particularly wires and cables, useful in the construction of devices such as high-field magnets, motors and generators. One such method is an external diffusion process which involves drawing niobium in a pure copper matrix to provide a multifilament niobium wire, plating tin on the copper surface of the wire and diffusing the tin into the copper matrix for reaction with the niobium. After coiling or otherwise forming the wire to preferred shape, the wire is heated to react the tin with the niobium and produce the superconducting Nb.sub.3 Sn phase in the wire. While the brittleness problem is thereby avoided during the forming or shaping stage, superconducting properties are degraded to the extent that the tin tends to form droplets (tin spots) on the wire surface during the diffusion-heating step. These tin spots remain as flaws and mechanically weak spots in the ultimate superconducting product.
Tin spots can be avoided in this process by applying the tin in incremental amounts in a number of separate plating steps with intermediate anneals to homogenize the materials. This procedure, however, is time consuming and expensive to carry out.
Another method which avoids tin spots, but does not require repeated plating steps and anneals, involves the use of bronze rather than copper as the matrix material. Reduction of the composite wire to final size, however, requires a number of draing operations and intermediate anneals making this process likewise time-consuming and expensive.